Regarding an improvement on clean-up rates of lean NOx catalyst, JP-A-5-133260 (1993), for instance, discloses a method for improving transient clean-up rates by changing a target air-to-fuel ratio from a rich condition to a lean condition and its reverse alternately. However, the above prior art does not teach nor suggest any reactivation (recovery) control of the clean-up rates to SOx poisoning based on time-elapsing effects of NOx catalyst by heating utilizing so-called after-burn efficiencies and the like.
Regarding conventional in-cylinder-injection engines, a prior art is described, for instance, in JP-A-4-241753 (1992). The technology is to make a fuel distribution homogeneous by controlling fuel injection timing based on a temperature of cooling water of the engine, in order to achieve stable stratified combustion in a lean condition of the in-cylinder-injection engine.
However, any special countermeasures for the exhaust gas of the stratified combustion, wherein the combustion is performed in a lean condition far from the stoichiometric air-to-fuel ratio, has not been considered. Regarding release of NOx, which is a particular problem of the lean combustion, no processing nor controlling has been considered.